Vacuum-tube making



Dec. 30, 1930.

VACUUM TUBE MAKING- Filed March 29, 1924 INVENTOR Patented Dec. 30, 1930 PATENT OFFICE FREDERICK S. MCCULLOUGH, OF WILKINSBURG, PENNSYLVANIA VAGU UH -TUBE MAKING Application led March 29, 1924. Serial No. 792,818.

This invention relates to the construction and manufacture of electron or vacuum tubes, especially those constructed for the handling or control of relatively large currents, such b' for instance, as the so-called power ytubes used in radio broadcasting stations, and relates particularly to tubes of the type shown in my co-pending application Serial No. 698,925, filed March 13, 1924.

In my said application, I have described 'a tube having an air cooled exterior anode. In said application, it is pointed out that, in making tubes having a metal envelope which is also the anode of the tube, it is necessary to heat the metal anode to a high degree of temperature for a sustained period of time to successfully de-gasify the tube, and that this is done largely by passing a current through the filament of the tube, whereby the anode is heated by radiation and electronic bombardment. vlhis recess is entirely satisfactory where a hig temperature lament is used. Y

It is well known in the art that certain substances, when applied to a filament, give a high electron emission at relatively low temperatures. rIhis means that, when such substance is applied to the filament of an electron tube, high emissivity is obtained at made use of in small tu s', that is, the socalled dry cell tubes.

However, little success has been attained in applying this knowledge to power tubes, although the desirability of tubes having a high electron discharge with .a low watt-hour consumption of current has been appreciated, especially for portable transmitting stations, and transmitting apparatus of small water craft. One of the chief diiculties in the way of using a low temperature filament has been A due to the diiculty of satisfactorily de-gasifying power tubes having the low temperature ilament, a problem which is not difficult to overcome 1n the small tubes.

The present invention has `for its object to provide a power tube having a filament capable of giving high electron emission with a low filament current, together with a meth- 50 od of treating the lament in the tube to comparatively low tem eratures. This is' initially bring it to a proper state of activation, that is, a state where high emissivity may be obtained without immediate disintegration of the filament.

A further object of the invention is to pro- |v vide a tube having an exterior air cooled anode which comprises also the shell of the tube and a low temperature cathode.

A further object of the invention is to provide a method of de-gasifying a tube having a low temperature cathode.

The inventlon may be readily understood by reference to the accompanylng drawing, in which the figure represents a longitudinal section through a tube of the type to which the invention is applicable.

In the drawing, 5 designates a metal tube having a closed endl 6. This tube is preferably made of nickel or some other metal which is passive to deteriorating oxidation atrelatively high temperatures.

Sealed to the open end of the tube, at 7, is a glass base or seal 8. This seal is developed by incorporating into the glass at the point of contact with the metal, an oxide, such as copper oxide, as described in my said copending application, and forming no part of the present invention.

Inside the glass seal or base is a cylindrical mounting or stem 9 which supports a filament so rod 10 that extends longitudinally of the tube, its upper end vbeing received in a quartz block 10a centered in the closed upper end of the tube. Having its upper end attached to this post, near the top thereof, isa ilament S5 or cathode 10b that spirals around the post 10 out of Contact therewith. The lower end of this lilament connects to the top of a short post or connector 10c well up inside the tube, so that the'iilament does not extend 90 down as far as the seal 7. Both postsv 10 and 10c extend through the glass, being sealed therein, and may have connectors 11 on the outer ends thereof.

Surrounding the stem 9 is a split band 12 95 having its ends clamped together at 12a to frictionally retain the band on the stem. This band carries a plurality of heavy wire supports 13, as described in my said application, lsuitably bent to shape, these wires lon extending longitudinally of the tube and having their u per ends tied together about the quartz bloc 10a. A continuous wire 14 coiled around posts 13, outside the filament,

forms the gridv of the tube.

At 15 is a conductor that passes through the glass and connects with the metal ring 12, thus providing a connector for the grid. At 16 is designated the tubular extension where the tube 1s connected with the pump, and where it is sealed oflt.

Initially secured to the metal shell 5 are one or moreheat radiatin fins or disks 17, which are in close roximity to the seal. I have shown two suc fins in full lines on the drawings. More could be provided, but as their presence before de-gasifying1 the tube.

would interfere with satisfactory eating to drive out the gas and condition the filament,

20 as hereinafter described, onl the two disks 17 are preferabl provided at st to protect the seal 7. Shou d it be desired, after the tube 1s evacuated, to have additional radiating surfaces, a sleeve 18, 'shown in dotted lines in the drawing, having fins 19, may be slipped over the completed metal shell after the tube has been completed.

The filament 10b is formed of molybdenum wire which has been alternately dipped in `a thorium nitrate solution and heated to about 700 degrees C., until a fairly heavy coating has been iformed on the wire. Such a fila-` ment, after being properly conditioned, as hereinafter descrlbed, will, when heated to a point approximatin a barely visible glow, produce an extremely igh electron emission. 'Other than the dipping of lthe filament, the present novel process begins after the tube has been assembled to the point shown 40' in the drawings, where the tube is ready to be evacuated.

The tube is connected to the pump in the usual manner at 16, and the greatest part of 'the gas in the tube is removed. Then, with the tube still on the ump, an oven is lowered around the tube an the tube is baked in the oven, at a temperature of about 450 de ees C., for a sustained period of time, an our probably being suilicient.. This drives most large amount of it out of the metal anode.

Then, an electric heating coil A yis lowered around the metal anode of the tube only, and a current passed therethrough to heat the anode, the temperature of ithe anode being raisedl by this means to about 800 or 1000 degrees C. This operation is continued for about one hour, and during that period lwhereby the glass will be kept from absorbing any of the gas liberated at this stage. This gas is,'of course, removed by the pump. During this operation, the anode is heated c; well beyond any temperature that will be -from' the anode,

of the gas from out of the glass base and a,

the oven'is retained around the entire tube,y

reached in the subsequent operation of the tu e.

The heating of the anode with the external coil A also causes the cathode or filament to be heated by radiation from the anode. This external aplication of heat to the cathode, in a 1 h vacuum, drives out gases from the cat ode and conditions the coating to withstand subsequent energization,'and brings it to a state where most favorable electron emission for a sustained period of time can be obtained.

Now that the tube is extremely hard, and the anode heated far above any temrature that will subsequently be reached 1n the later operation 'of the tube, and the filament well conditioned or activated, a current is passed through the filament which is well below the normal operating current.

This is done with the external coil and oven around the tube. Gradually, the filament current is increased until, eventuall the normal operating current is a plied. very minute quantity of gas may be liberated by the passage of currents through the filament, but this is immediately removed by the pump. The normal operating voltage is reached gradually in order to avoid any initial excessive'straining of the filament.

When the normal operating current is applied to the filament, and with the external 'anode still heated, a positive potential is applied to the anode, andthe anode subjectedto electronic bombardment for a riod of time. This bombardment, takin place with the anode at a temperature wel above any temperature which 1t` will subsequently reach in later operation, effectively removes all traces of as that 'might otherwise be liberated in t e later operation of the tube.

In order to more effectively bombard the plate, and de-gasify the grid, which, like the filament, is initially heated by radiation the tube may be connected to oscillate for a while. Then'it ma be further cleared up by s arkin high vo ta e currents through it. fter t is, 1t may e again operated with a normal filament voltage and a positive potential on the plate, then sealed off, and removed from the pump.

In this manner, a tube having a low temperature cathodezis provided, which tube is extremely free of gas. The filament, being conditioned' or activated in the tube and in a relatively high vacuum, gives very satisfactoryl results. Were a current to be passed through the filament before the filament had been' conditioned in the tube inv this manner, or before the tube is hard, it would not be so satisfactory vor 'so durable, and would have to be operated at higher tem- -peratures, and would be subject to disintegraheated during the exhaustion of the tube,

while fins 19 are applied after the tubeis com leted.

A though l have described a particular filament construction, I do not confine myself to the use of molybdenum as a base or thorium nitrate as a coating, though both are preferred.

`I claim as'my invention:

1. The method of preparing a high emitting low temperature cathode in electron tubes having metal ,shells with glass seals, which consists in assembling into the tube a cathode having a lowtemperature high electron emitting coating thereon, connecting the assembled tube with a pump, baking the entire tube tok partially de-gasify it at a moderately high temperature but below the melting point of glass, then raising the temperature of the metal shell of the tube to a glowing temperature while maintaining the glass seal hot but below its melting temperature, the tube remaining in connection with the pump, and the cathode being'heated in a vacuum b the radiation of heat from the shell, whereby the cathode is conditioned for service and de-gasified in a vacuum and in place in the tube. l

2. rlhe method of manufacturing electron tubes having a metal envelope forming the anode of the tube, a glass seal for the envelope and a coated hlgh emission low temperature cathode which consists in assembling the tube with alcoated cathode therein, exhausting the tube, and, during the process of exhaustion, baking the entire tube and subjecting the 'envelope by an external heating means to a temperature considerably above that to'which, the glass is subjected land ranging in proximity to 1000 degrees Q., the cathode also being heated at this time 'by radiation from the envelope, then, while continuing the exhausting operation and with the continued application of heat to the tube sufficient to keep the envelope above its subsequent normal operating temperature, subjecting the anode to electron `bombardment from the ilament, and subsequently sealing od the tube.

3. rlhe method of conditioning electron tubes having metal envelopes and glass seals and provided with low temperature high i emission cathodes, in which cathode there is a metalbase and a coating, which consists in v metal envelope to the glass seal in a suilicient amount to injure the said seal, then connecting positive current to the metal envelope to bol'nb'ardthe tube, subsequently oscillating the tube while .maintaining the tube connected to the pump, whereby the tube is degasiied and conditioned for service.

4. The method of conditioning for service electron tubes having metal envelopes and glass seals, and provided with cathodes havmg a high emission low tem erature coating which consists in assemb ing the4 coated cathode in the tube, exhausting the tube, and while the exhausting operation is being continued heating the tube and heating the cathode from an external heating means, and raising the tem erature of the cathode thereby to about 800 egrees C. or above, and the temperature of the metal envelope above the melting point of glass and at the same time maintaining the temperature of the glass seal below the melting point of glass, then bombarding the tube by applying current to the metal envelope, subsequently passing high potential sparking current through. the tube while connected to the pump, whereby the tube is degasified and conditioned for service.

5. The method of conditionin for service an electron tube provided wit a cathode having a high emission low temperature coating and a metal anode sealed to a glass en velope which consists in assembling the coated cathode in the tube, exhausting the tube, and while the exhausting operation is being continued heating the tube and 'heating the cathode from an external heating means, raising the temperature of the anode above the melting point of the glass and atthesame time maintaining the temperature of the glass seal below the melting point of glass, and gradually raising the temperature of the cathode to about 800 degrees C. or above, then-bombarding the tube by passing a current through the cathode while the tube remains heated, subsequently causing the tube to oscillate, then again bombarding the tube and tinally sealing the same off for normal'use.

6. r)The method of manufacturing electron tubes having a metal envelo e forming the anode of the tube, a glass seal or the envelope and a coated high emission low temperature cathode, which consists in assembling the tube with a coated cathode therein, exhausting the tube, and, during the process of exhaustion, baking the entire tube and subjecting the envelope by an external heating means to a temperature considerably above that to which the glass is subjected and ranging in proximity to 1000 degress C. while preventing injury to the glass seal, the cathode also belng heated at this time by radiation from` temperature high emission cathode which consists in coating a. metal base with thorium nitrate, dr ing such coating on the base, mounting tiie coated base in a tube to rovide a cathodetherefor, exhausting the tu and, i

when the tube is extemely well evacuated but while the process of exhaustion is continued, heating the tube and the cathode from an external heatin means, raising the temperature of the cat ode by such means to about 800 degrees C. and raising the temperature of the anode above the melting .point of glass and at the same time maintaimn the temperature of the seal between the g ass and the anode below the melting point of glass, then bombarding the tube by applying current to the metal envelope, subse nenti connecting sparking currentto the tiibe and7 again bombardin the tubez and sealing the tube o the pump or operatlon.

In testimony whereof I hereunto aiiix my signature.

FREDERICK s. MocULLoUGfH.` 

